1. Field of the Invention
The present invention generally relates to rubidium atom oscillators and, more particularly, to a rubidium atom oscillator used for a reference frequency source for broadcasting, a clock source established in a subordinate office of a lower part of a main office which constitutes a digital synchronous network or a clock source provided in a base station of mobile communications.
In recent years, in the market of a source of reference frequency, there is a demand for a small, low cost, high performance rubidium atom oscillator. In order to realize a rubidium atom oscillator that satisfies such requirements, simplification of circuits and selection of low cost parts of a rubidium atom oscillator are advanced. Consequently, as for a voltage controlled crystal oscillator (VCXO) used as a slave oscillator, a small general-purpose rubidium atom oscillator having a large manufacturing deviation and a large degradation of an output frequency has been used. Accordingly, it is desired to make an improvement with respect to stability in frequency or influence of circumference noise.
2. Description of the Related Art
FIG. 1 shows a composition of a conventional rubidium atom oscillator. The rubidium atom oscillator shown in FIG. 1 comprises: a voltage controlled crystal oscillator (VCXO) 80 which oscillates a frequency of about 10 MHz; a frequency synthesis part 81; a low-frequency oscillator 82; an atomic resonator 83 which uses rubidium atoms; a preamplifier 84; a synchronous wave detector 85; an alarm (ALM) circuit 86 which generates an alarm when a resonance signal output from the preamplifier 84 is not detected; a sweep circuit 87 which generates a sweep waveform to VCXO 80; a switching circuit 88 which is controlled by an output of the alarm circuit 86 so as to select a synchronized signal from the synchronous wave detector 85 when an alarm output is detected and to select a sweep signal from the sweep circuit 87 when the alarm output is not detected; and an integrator 89 which integrates the output of the switching circuit 88 so as to generate a control voltage for VCXO 80.
A description will now be given of an operation of the above-mentioned rubidium atom oscillator. An output of VCXO 80 is supplied to outside as an output (Rb-OSC) of the rubidium atom oscillator, and also supplied to the frequency synthesis part 81. The frequency synthesis part 81 synthesizes and multiplies the output frequency of VCXO 80 so as to generate the atomic resonance frequency (6.8346282 . . . GHz). Moreover, the frequency synthesis part 81 performs phase modulation by an output signal of the low-frequency oscillator 82. The low-frequency oscillator 82 oscillates a frequency of 100-200 Hz. The output of the frequency synthesis part 81 is supplied to the atomic resonator 83.
FIG. 2 shows the internal composition of the atomic resonator 83. As shown in FIG. 2, the atomic resonator 83 includes: a magnetic shield case 83xe2x80x2 accommodating the atomic resonator; a lamp house 830 accommodating a rubidium lamp 832; a high-frequency source 831; a cavity 833 which constitutes a cavity resonator; a resonance cell 834 in which rubidium atoms (gas) are filled; a photodiode 835 which detects a rubidium light; and a microwave excitation antenna 836.
An atomic resonator is accommodated in the magnetic shield case 83xe2x80x2. The lamp house 830 and the cavity 833 are temperature-controlled at 90 degrees and 70 degrees, respectively. The rubidium lamp 832 provided inside the lamp house 830 emits a light by electrodeless discharge caused by high-frequency excitation of rubidium atoms (gas) being carried out by the high-frequency source 831. The cavity 833 is tuned to the atomic resonance frequency (=6.8346 . . . GHz), and the microwave output from the frequency synthesis part 81 (refer to FIG. 1) is emitted from the microwave excitation antenna 836. The microwave is irradiated to the rubidium atom enclosed in the resonance cell 834. The photodiode 835 detects the light of the rubidium lamp 832 which passed through the resonance cell 834. If the frequency of the microwave irradiated to the rubidium atoms matches the resonance frequency of rubidium atom, an amount of light received by the photodiode decreases due to a light-microwave double resonance, thereby, generating a resonance signal (a reduction in the amount of light is regarded as a detection of a resonance signal).
Returning to FIG. 1, the preamplifier 84 amplifies the output of the photodiode 835. The amplified output is supplied to the synchronous wave detector 85 as an atomic resonance output, and also supplied to the alarm circuit 86. Based on existence of the resonance signal in the output of the atomic resonator 83, the alarm circuit 86 distinguishes the states of frequency lock and unlock, and outputs an alarm signal to outside. The switching circuit 88 switches the signal to be supplied to the integrator 89 according to the alarm signal. That is, the switching circuit 88 selects the output of the synchronous wave detector 85 in a non-alarm state in which the resonance signal is detected. On the other hand, the switching circuit 88 selects an output of the sweep circuit 87 which generates a voltage which carries out the sweep of the output frequency of VCXO 80 in a state in which the resonance signal has not been detected. The output of the switching circuit 88 is supplied to the integrator 89. The integrator 89 integrates the input signal, and changes the input signal into a control signal.
The synchronous wave detector 85 carries out synchronous detection of the resonance signal generated by the atomic resonator 83 by the output frequency of the low-frequency oscillator 82, i.e., the same frequency as the phase modulation in the frequency synthesis part 81. The integrator 89 smoothes the output of the switching circuit 88 into a direct-current signal, and outputs the directcurrent signal as an error signal. By applying the error signal output from the integrator 89 to VCXO 80 as a frequency control voltage, the output frequency of VCXO 80 is kept equal to the resonance frequency of rubidium atoms with respect to stability of frequency (a frequency lock is carried out).
As mentioned above, VCXO is used for the conventional rubidium atom oscillator. Since VCXO enables a frequency variable by an external control voltage, a change in the frequency of VCXO, which is caused by a change in an outside environment, such as temperature, a power supply, and noise, or aging, is large as compared with the crystal oscillator (XO) of a fixed frequency output. Such a characteristic change is especially large in a general-purpose small VCXO that has come to be used in recent years. FIG. 3 is a graph showing changes in the characteristics of VCXO and XO with passage of time. In FIG. 3, a horizontal axis expresses lapsed days (day), and a vertical axis expresses a rate of change in frequency (xcex94f/f0). Af is a change in frequency and f0 is a basic frequency of crystal oscillators. It can be appreciated from the graph of FIG. 3 that the change in the characteristic of VCXO with the passage of time is larger than that of XO.
In order to correct such a frequency change and aging of VCXO, generally, a more steep frequency variable characteristic is given to VCXO. For this reason, the frequency stability of VCXO tends to be influenced by a circumference noise, etc. Therefore, when a rubidium atom oscillator is constituted using VCXO, there is a problem in that VCXO becomes a major cause of a characteristic degradation such as degradation in the short-term stability (a rate of stabilization within a short time) of a rubidium atom oscillator or phase noise degradation (instability due to phase change).
Furthermore, since an amount of change with the passage of time is large, the frequency of VCXO is swept during a period (about 10-30 minutes) until a frequency lock is carried out at the time of starting or when it becomes impossible to detect the resonance signal due to a certain failure, i.e., at the time of alarming a frequency unlock. FIG. 4 is a graph showing the frequency change at the time of alarming and a frequency lock. In FIG. 4, a horizontal axis expresses time, and a vertical axis expresses a rate of change in frequency (xcex94f/f0). As shown in FIG. 4, at the time of starting and occurrence of a failure, the modulation sensitivity of VCXO with respect to the sweep voltage is large in the alarming state (non-detection state of the resonance signal). Therefore, the frequency changes sharply and the frequency is locked in the non-alarming state (detection state of the resonance signal). Thus, there is a problem in that the frequency stability of a rubidium atom oscillator deteriorates remarkably at the time of alarming.
It is general object of the present invention to provide an improved and useful rubidium atom oscillator in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a rubidium atom oscillator which is not influenced by a circumference noise or the like, and is excellent in the short-term stability and the phase noise characteristic.
Another object of the present invention is to provide a rubidium atom oscillator which can minimize degradation in the frequency stability in a frequency unlock state.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a rubidium atom oscillator comprising: a crystal oscillator which oscillates a fixed frequency as an atomic resonance frequency; a direct digital synthesizer which inputs an output of the crystal oscillator as a system clock and also inputs tuned data corresponding to an error signal generated according to a resonance frequency so as to carry out a variable control of an output frequency; a frequency synthesizer which synthesizes and multiplies an output of the direct digital synthesizer and applies a phase modulation with a low-frequency signal; an atomic resonator which inputs an output of the frequency synthesizer and detects an error signal with respect to a resonance frequency of rubidium atoms; a tuned-data generating circuit which inputs the error signal from the atomic resonator so as to generate the tuned data corresponding to the error signal, wherein the output frequency of the direct digital synthesizer is output from the rubidium atom oscillator.
In the rubidium atom oscillator according to the present invention, the tuned-data generating circuit may include a data adder which inputs and sums a digital signal corresponding the error signal from the atomic resonator and an output signal of an erasable programmable read only memory that stores the tuned data output to the direct digital synthesizer so as to output the tuned data in which the error signal is reflected.
Additionally, the rubidium atom oscillator according to the above-mentioned invention may further comprise a temperature correction circuit which generates a temperature correction signal for correcting a change in the resonance frequency of the atomic resonator based on a temperature detection signal representing a circumference temperature of the atomic resonator, wherein the temperature correction signal is input to the data adder of the tuned-data generating circuit.
Alternatively, the rubidium atom oscillator according to the present invention may further comprise a temperature correction circuit which generates a temperature correction signal for correcting a change in the resonance frequency of the atomic resonator based on a temperature detection signal corresponding to a collector voltage of a transistor which controls an electric current supplied to a heater provided in the atomic resonator, wherein the temperature correction signal is input to the data adder of the tuned-data generating circuit.
Additionally, the rubidium atom oscillator according to the present invention may further comprise a light amount correction circuit which generates a light amount correction signal for correcting a change in the resonance frequency of the atomic resonator based on a light amount detection signal representing a change in an amount of light of a rubidium lamp provided in the atomic resonator with respect of passage of time, wherein the temperature correction signal is input to the data adder of the tuned-data generating circuit.
In the rubidium atom oscillator according to the present invention, the data adder may input a signal representing a variable control voltage from outside so that the tuned data output from the tuned-data generating circuit is variable to change the oscillation frequency of the rubidium atom oscillator.
The rubidium atom oscillator according to the above-mentioned invention may further comprise an analog-to-digital converter which converts the signal representing the variable control voltage into a digital signal and supplies the digital signal to the data adder.
The signal representing the variable control voltage may be a digital signal so that the digital signal is directly supplied to the data adder.
Additionally, there is provided according to another aspect of the present invention a rubidium atom oscillator comprising: a crystal oscillator which oscillates a fixed frequency as an atomic resonance frequency; a direct digital synthesizer which inputs an output of the crystal oscillator as a system clock and also inputs control data corresponding to an error signal generated according to a resonance frequency so as to carry out a variable control of an output frequency; an atomic resonator which inputs an output of the direct digital synthesizer after being subjected to a predetermined process and detects an error signal with respect to a resonance frequency of rubidium atoms; a control-data generating circuit which inputs the error signal from the atomic resonator so as to generate the control data corresponding to the error signal, wherein the output frequency of the direct-digital synthesizer is output from the rubidium atom oscillator.
According to the present invention, the rubidium atom oscillator can be constituted using the crystal oscillator having a fixed frequency output, as a source of generation of an atomic resonance frequency, which cannot be easily influenced by a circumference noise or the like. Thereby, the rubidium atom oscillator excellent in the phase noise characteristic and frequency short-term stability of an output can be achieved.
Moreover, since the crystal oscillator does not have any frequency variable element and an output frequency deviation and a change with passage of time (aging) are small, a resonance signal can be detected without sweeping the input frequency to the atomic resonator at the time of a frequency unlock. Therefore, there is less degradation of the frequency stability at the time of alarm than a conventional rubidium atom oscillator, and a miniaturization and cost reduction can be achieved by omitting a sweep circuit.
Further, since the tuned data of the direct digital synthesizer is controlled to correct a fluctuation in the circumference temperature and a change in an amount of light of the rubidium lamp, the rubidium atom oscillator which is excellent in the temperature characteristic and the aging characteristic can be achieved.
Other objects, features and advantages of the present invention will become more apparent from the scope of the present invention when read in conjunction with the accompanying drawings.